The small mammals on the Channel Islands off the coast of California are the subject of this study on genetic divergence under conditions of isolation. Using starch gel electrophoresis and breeding data, alleles at 28 genetic loci have been identified. This genetic variability and intraspecific divergence of endemic subspecies of the deer mouse, Peromyscus maniculatus, on four of the islands have been analyzed. These deer mice have relatively high levels of polymorphism for insular populations. Selection seems to be operating in the maintenance of some of the loci, for there is great variance in their apparent inbreeding coefficients. The mean heterozygosity per individual varies in the populations, being much higher on one of the islands than the others. Nei's measure of genetic distance between groups compared on the basis of electrophoretic variants is used. His estimate of time of divergence of these groups, based on genetic distance, is applicable particularly to closely related groups. The length of time each island population has been isolated from the others is calculated and found to be consistent with the periods of isolation estimated on the basis of geological data. Populations of deer mice on the remaining four Channel Islands are currently under investigation, and an analysis of differentiation between all the islands will be made when all populations have been sampled.